An amorphous material is a solid in which the atoms exhibit no long range order and in which the internal structure is irregular or random, as opposed to a crystalline material, which has a regular repeating internal structure. An example of an amorphous material is ordinary window glass, which is formed when molten silicate is cooled and acquires high viscosity, without allowing a regular crystal lattice to form. The amorphous state of the glass results in various useful optical properties, such as its transparency. The presence of various ingredients (e.g., Na, Ca, B, Pb in addition to silica) may have a significant influence on the final properties of the amorphous material (e.g., its color, transparency, softening point or glass transition temperature, Tg).
Group-III metals of the periodic table (i.e., aluminum, gallium and indium) can form semiconductor compounds with group-V elements of the periodic table (e.g., nitrogen, phosphorous and arsenic). For example, group-III metals can form arsenide materials, such as gallium arsenide (GaAs), or phosphide materials, such as gallium phosphide (GaP). GaAs is a semiconductor widely used, for example, in microwave frequency integrated circuits, light emitting diodes and solar cells. GaP is used, for example, in red, orange and green light emitting diodes.
Group-III metals can also form nitrides by interacting with nitrogen (N), i.e., aluminum nitride (AIN), gallium nitride (GaN) and indium nitride (InN). Group-III metal nitrides are semiconductors having various energy gaps (between two adjacent allowable bands), e.g., a narrow gap of 0.7 eV for InN, an intermediate gap of 3.4 eV for GaN, and a wide gap of 6.2 eV for AIN.
Solid group III-V semiconductor materials have an ordered crystalline structure, giving them advantageous chemical and physical properties, such that electronic devices made from group III-V material can operate at conditions of high temperature, high power and high frequency. Electronic devices made from group III-V material may emit or absorb electromagnetic radiation having wavelengths ranging from the UV region to the IR region of the spectrum, which is particularly relevant for constructing light emitting diodes (LED), solid-state lights and the like. Amorphous group III-V materials have certain useful optical properties, and may be employed in a wide variety of additional applications, such as solar batteries and full color displays.
Techniques for preparation of amorphous materials include: rapid solidification, thin-film deposition processes (e.g., sputter deposition and chemical vapor deposition), and ion implantation. In rapid solidification, the amorphous material is produced directly from a liquid melt, which is cooled very quickly such that there is insufficient time for an ordered crystal structure to form. Thin-film deposition involves depositing a thin film onto a substrate, or on previously deposited layers on the substrate.
Sputter deposition is one type of thin film deposition technique. The atoms in a solid target material are ejected into a gas phase by ion bombardment. Each collision knocks off additional atoms, where the number of ejected atoms per incident ion (i.e., the sputter yield) is dependent on several factors, such as the energy of the incident ions, the respective masses of the ions and atoms, and the binding energy of the atoms in the solid. The ions are provided by a plasma, usually of a noble gas (e.g., argon). The ejected atoms are not in their thermodynamic equilibrium state, and tend to deposit on all surfaces in the vacuum chamber. Therefore a substrate in the chamber will end up being coated with a thin film having the same composition of the target material. The target can be kept at a relatively low temperature during sputter deposition, since no evaporation is involved. In reactive sputtering, the plasma gas includes a small amount of a non-noble gas, such as oxygen or nitrogen, which reacts with the material after it is sputtered from the target, resulting in the deposited material being the product of the reaction, such as an oxide or nitride.
Chemical vapor deposition (CVD) is another type of thin film deposition, where the film is formed by a chemical reaction. The substrate is exposed to a mixture of gases, which reacts with the substrate surface to produce the desired deposit, which condenses on the substrate. CVD is usually performed at high temperature in a furnace, or in a CVD reactor in which the substrate is heated. Unwanted reaction byproducts are usually produced in the reaction, which are removed by gas flow through the reaction chamber. Plasma may be used to enhance the rates and lower the temperatures of chemical reaction. Metal-organic chemical vapor deposition (MOCVD) involves organo-metallic compounds as the reactants, which reduce reaction temperature in comparison to ordinary CVD sources.
Ion implantation involves implanting ions of a first material in a second target material. The ions are electrostatically accelerated to a high energy, before impinging on the target material, such as on the surface of a substrate. The amount of material implanted, known as the dose, is the integral over time of the ion current. By controlling the dose and the energy, it is possible to change and disrupt the crystal structure of the target surface in such a way that an amorphous layer is formed. The impinging ions break chemical bonds within the target material, and form new bonds which are unorganized and not in thermodynamic equilibrium, resulting in the target material becoming amorphous.
An article entitled “Compositional and Structural Studies of Amorphous GaN Grown by Ion-assisted Deposition” to Lanke et al. (Material Research Society Symposium 2002), is directed to a method for growing amorphous GaN using an energetic nitrogen ion beam. Amorphous GaN films were prepared by electron beam evaporation of gallium metal in the presence of an energetic nitrogen ion beam (ion-assisted deposition). The films were deposited at room temperature using incident nitrogen ion energies in the range 40-900 eV. Compositional analysis was carried out on films grown on silicon and glassy carbon substrates. The analysis showed that the grown films can reasonably be considered amorphous GaN. Films deposited with nitrogen ion energy of around 500 eV are transparent across the visible, whereas lowering the ion energy below 300 eV caused the films to become progressively opaque.
U.S. Pat. No. 5,976,398 to Yagi, entitled “Process for manufacturing semiconductor, apparatus for manufacturing semiconductor, and amorphous material”, is directed to an amorphous nitride III-V compound semiconductor, and an apparatus and process for its manufacture. The manufacturing process utilizes plasma-enhanced MOCVD. The semiconductor manufacturing apparatus includes a reactor, a first and second activation-supply portions, an exhaust pipe, a heater, and a substrate holder. The substrate holder holds a substrate inside the reactor, which is allowed to form a vacuum. Each activation-supply portion is composed of a pair of gas introducing pipes, a quartz pipe connected with the reactor, and a microwave waveguide (or alternatively, a radio frequency coil) for providing activation.
Plasma of a V group element (e.g., nitrogen plasma) is generated at the first activation-supply portion and introduced into the reactor. For example, N2 gas is introduced from the gas introducing pipe, and a microwave oscillator supplies microwaves to the microwave waveguide, which induces a discharge in the quartz pipe and activates the N2 gas. A metal organic compound containing a III group element (e.g., Al, Ga, In) is supplied by a gas introducing of the first activation-supply portion. An auxiliary material (e.g., He, Ne, Ar, H2, Cl2 FI2) is supplied by the gas introducing pipe of the second activation-supply portion. The auxiliary material (e.g., hydrogen plasma) reacts with an organic functioning group of the metal organic compound, including the III group element, to inactivate the organic functional group. The vaporized metallic organic compound and the plasma of the auxiliary material is added to the plasma of the V group element.
The heater heats the substrate to the appropriate temperature (e.g., from 200° C. to 400° C.). A film of amorphous material, containing the III group element and the V group element, is formed on the substrate. The film of the semiconductor compound contains the III group element and the V group element. For example, the amorphous material is hydrogenated amorphous gallium nitride. The amorphous material is suitable as an optical semiconductor for optoelectronic applications.
US Patent Application Pub. No. US 2002/0100910 to Kordesch, entitled “Band gap engineering of amorphous Al—Ga—N alloys”, is directed to an amorphous semiconductor alloy including aluminum and gallium, and a method for its production, which utilizes sputter deposition. A semiconductor substrate is positioned on an anode inside a reactive sputter deposition chamber. The sputter deposition chamber also includes a sputter target on a target cathode. The sputter deposition chamber is coupled with an RF source and a matching network. The sputter target contains aluminum and gallium (e.g., a single integrated target with both aluminum and gallium, a single target with an aluminum portion and a gallium portion, or discrete targets of aluminum and gallium). The sputter target may also contain indium. Nitrogen gas is introduced into the sputter deposition chamber. The sputter deposition chamber is operated to promote reaction of the aluminum and gallium of the sputter target with the nitrogen. The semiconductor substrate is maintained at a deposition temperature (e.g., between about 77K to about 300K), selected to ensure that the grown alloy is amorphous. The relative proportions of aluminum and gallium are selected such that the amorphous alloy will have a band gap between about 3 eV and about 6 eV. The amorphous alloy has the chemical formula: AlxGa1-xN. The amorphous alloy may be doped, such as with a rare earth luminescent center, for various photonics applications.
A quantum dot (QD) is a nano crystal of a semiconductor material, spatially surrounded by material of a different conduction-phase (i.e., conducting, semiconducting or dielectric material). In a quantum dot, an exciton (i.e., an electron-hole pair) may be formed by excitation of a valence electron. A quantum dot represents a quantum mechanical “atom-like” structure, in which electrons can be excitated to very few higher (non-bonding) orbitals, leaving “holes” behind them. The excitation of electrons in a quantum dot may be electric excitation, photonic excitation, or thermal excitation.
In traditional quantum dot structure, nano crystals (i.e., QDs) of lower band gap energy (Eg) are embedded within a crystalline structure matrix of a material having higher Eg. For example germanium QDs may by embedded within a silicon crystal. Alternatively, nano-crystal QDs may be embedded within an amorphous matrix. In this case, one can use a single chemical material constructing a two-phase composite material. Thus, the difference in band-gap energies between the QDs and the matrix material is derived from the two phases, namely the crystalline of the QDs and the amorphous of the surrounding matrix. Having identical ingredients, a crystalline phase generally has a lower Eg than the amorphous phase. For example, this would be the case in GaAs QDs embedded within a GaAs amorphous matrix.
In a case where two chemical substances are involved in the composite material, further versatility rises. The crystalline phase of the QDs is less tolerant to material composition, whereas the amorphous one is more tolerant. In this case, one can get QDs of different band-gap energy conditions, namely QDs having lower Eg within a higher Eg matrix (i.e., as in the usual case described above), or QDs having higher Eg within lower Eg matrix. Such amorphous matrix having nano-crystallites embedded therein is referred to as “Amorphous-Nano-Crystalline” (ANC). An example for such matter would be the InGaN system.
In the case of a single chemical substance, the band gap energy of a superiorly amorphous material (i.e., inhibiting particle order on the scale of nanometers or shorter) is higher than an ANC material of the same substance (i.e., an ordered more crystalline material has a lower Eg than its amorphous equivalent). In a mixed phase composite material, the nano crystalline phase may be of different composition than the surrounding amorphous phase. For example, in the material system of InGaN, there might be more indium in the crystalline phase (i.e., less indium in the amorphous one) or vice versa, less indium in the crystalline phase (i.e., more indium in the amorphous phase). In the latter case one may obtain a larger Eg in the crystallites and a smaller Eg in the amorphous matrix.
An ANC layer may be wafered or stacked between two superiorly amorphous layers. The superiorly amorphous layer may have a band gap energy (hereinafter, “Eg”) value smaller or greater than the Eg of the nano-crystallites. Therefore, in the case of greater Eg, free charge carriers would be spatially trapped within the ANC layer, since they would not have sufficient energy to pass the superiorly amorphous layer energetic barrier. In the case of smaller Eg, free charge carriers would be spatially “released” upon generation. When each of the capping superiorly amourphous layers is doped with n-type and p-type materials, respectively, a quantum dot p-i-n junction is obtained. In such a quantum dot p-i-n junction, free electrons and holes of the ANC layer may combine and emit a photon, or migrate toward the n-type doped and p-type doped amorphous layers, respectively, if radiated excessively. If the Eg of the ANC layer is smaller than the Eg of the amorphous layer, free charge carriers formed by photons within the amorphous layer may easily “escape” to the surrounding layers (e.g., amorphous material doped with p-type material). If electrodes are coupled to each of the amorphous layers, then a quantum dot p-i-n junction, yielding a quantum dot electronic device may be obtained. If the electrodes become electrically charged by an external power supply, then electric current may be generated through the quantum dot p-i-n junction, and a light emitting device may be employed.
A quantum dot electronic device may be employed as a photovoltaic (PV) cell. In a PV cell, free charge carriers (i.e., electrons and holes) are produced by the photovoltaic effect, and are induced to migrate under an electric field of a p-i-n junction, toward n-type and p-type doped caps. The photovoltaic effect takes place when a photon hits the PV cell, having a wavelength, equal to (or higher than) the band gap of a semiconductor material constructing the PV cell. That photon may be absorbed by an electron, pumping the electron from the valence band to the conduction band, leaving a hole and thus forming an exciton. Since the PV cell is capped within a p-i-n junction, the electron and the hole may migrate in the electric field of the junction, in opposite directions. If electrodes are coupled to each end of the p-i-n junction, then the electrodes become electrically charged. When the quantum dot PV cell is connected to an external circuit, an electric current may be generated between two electrodes on each side of the PV cell.
A PV cell may include a plurality of semiconductor materials, each having a different value of band gap energy, and absorbing photons of a different wavelength. In that case, the quantum dot PV cell may be employed as a solar cell, absorbing a plurality of wavelengths from the broad spectrum light of sunlight, and turning it into electricity. Quantum dot solar cells are known in the art.
U.S. Pat. No. 6,566,595 to Suzuki, entitled “Solar Cell and Process of Manufacturing the Same”, is directed to a solar cell employing a quantum dot layer in a p-i-n junction. The solar cell includes a p-type semiconductor layer and an n-type semiconductor layer made of a first compound semiconductor material. At least one quantum dot layer is formed between the p-type semiconductor layer and the n-type semiconductor layer. The quantum dot layer is constructed of a second compound semiconductor material and has a plurality of projections (i.e., quantum dots) on its surface. The quantum dots are of different sizes on a single quantum dot layer, or on any one of the quantum dot layers.
The quantum dot layer is inserted in the i-type semiconductor layer of the p-i-n junction. Thus, light of wavelength corresponding to the practical forbidden band width of the quantum dot layer is absorbed, in addition to light of wavelength corresponding to the forbidden band width of the semiconductor material forming the p-n junction. This increases the photoelectric conversion efficiency of the solar cell. The forbidden band width of the quantum dot layer can be varied depending on the combination or compound crystal ratio of the semiconductor used for forming the quantum dot layer. Thus, the wavelength range in which the photoelectric conversion can be carried out may be extended, and a solar cell which allows photoelectric conversion of varying wavelengths at high efficiency corresponding to the incident light can be manufactured. In a process of manufacturing the solar cell according to Suzuki, the quantum dot layer may be formed by lithography and selective etching, or by self-growing mechanism. The semiconductor material used for forming the quantum dot layer may be a compound of a group III element and a group V element shown in the periodic table, such as InGaAs or GaAs.
US Patent Application Pub. No. US2005/0155641 to Fafard, entitled “Solar Cell with Epitaxially Grown Quantum Dot Material”, is directed to a photovoltaic solar cell having a subcell structure, and to a method for making such a solar cell. The solar cell is a monolithic semiconductor photovoltaic solar cells including at least one subcell, having a self-assembled quantum dot material. Each of the subcells of the solar cell exhibits a different bandgap energy value, and thus absorbs photons of different wavelengths. The subcells are disposed in order of increasing effective band gap energy, with the subcell having the lowest effective band gap energy being closest to the substrate. A barrier semiconductor layer is formed between each pair of subcells of the solar cell.
The method for making the solar cell includes epitaxial growth of the quantum dot material. The growth temperature of the quantum dot layers is used to adjust the shape and composition of the quantum dots. The temperature during the overgrowth of the barrier of each quantum dot layer may be varied at different stages of the overgrowth, to further control the size and composition of the quantum dots and therefore the absorption characteristics of self-assembled quantum dot material. The combination of epitaxial growth parameters is chosen to obtain quantum dot layers having a high in-plane density of highly uniform quantum dots having desired energy levels. Such growth parameters are: growth temperature, the group-V over-pressure or the III/V ratio, the quantum dot material, the amount of material used to obtain the self-assembled growth transition between a uniform quasi two-dimensional film to three-dimensional islands, the growth rate or the pauses used during the growth, and the overgrowth conditions such as growth temperature and growth rate.